In oil field recovery operations, a casing, in the form of a steel pipe, or the like, is often placed in an oil and gas well to stabilize the well bore. In these installations, a cement sheath is formed in the annulus between the casing and the wall of the well bore to support the casing, to prevent migration of fluids in the annulus, and to protect the casing from corrosive formation fluids.
In accordance with conventional cementing operations, the sheath is formed by introducing a cement slurry into the upper end portion of the casing at the ground surface and allowing the cement to flow through the casing to the bottom of the well and reverse direction into the annulus. The cement then flows into and through the annulus between the casing and the wall forming the well, and circulates back to the ground surface. The flow of cement is then terminated and the cement allowed to set to form the sheath.
Numerous challenges can be present in these types of cementing operations. For example, it is often difficult to obtain the proper circulation of cement inside the annulus due to a weak formation around the well. Also, the hydrostatic weight of the cement exerts significant pressure against the formation, especially when additional pressure is applied to the formation due to the friction of the cement slurry that must be overcome.
One technique utilized to overcome these deficiencies and reduce the formation pressure employs reverse circulation in which the cement slurry is pumped down the annulus and back up the casing. While this greatly reduces the total pressure applied to the formation, it has several drawbacks. For example, it is impossible for the operator to determine exactly when the cement completely fills the annulus without the use of some type of tool which is expensive and time consuming. Thus, the operator runs the risk of either not completely filling the annulus with the cement or of filling the cement back up inside the casing string, thus covering potential productive areas and/or requiring additional time and expense to drill out this cement.
Another challenge to reverse circulation is that the heavier cement tends to flow inside the casing due to “U-Tubing.” Since typical float equipment used to prevent this cannot be used in reverse circulations, pressure must be held on the annulus until the cement has sufficiently set to prevent the U-Tubing. This can cause a micro-annulus to form between the cement sheath and casing. A micro-annulus can make it difficult to bond log the casing to evaluate the quality of the cementing operation and determine if the annulus is properly sealed. A micro-annulus can also allow unwanted flow of gas, brine, etc., behind the casing.
Still further, since the cement will not vary much in density throughout the height of the well bore, the benefit of reverse circulating a conventional cement is minimal since the total hydrostatic pressure of the cement column acts on the formation at the end of the operation.
Therefore, what is needed is a system and method that eliminates the problems with conventional circulation, yet avoids the problems associated with reverse circulation.